Heretofore, a rechargeable secondary battery has been continuously studied as a battery that is usable economically for a prolonged period of time. In particular, a lithium battery has advantages such as high output and high energy density and is usually composed of positive and negative electrodes each having an active material capable of reversible extraction/insertion of a lithium ion, and a nonaqueous electrolyte.
As the positive electrode active material of such a lithium ion battery, for example, a metal oxide, a metal sulfide or a polymer is used, and known examples include lithium-free compounds such as TiS2, MoS2, NbSe2 and V2O5, and a lithium composite oxide such as LiMO2 (M=Co, Ni, Mn, Fe or the like) and LiMn2O4. Among these, a battery fabricated using LiMn2O4 has a high battery capacity but in consideration of, for example, capacity deterioration during high-temperature storage and dissolution of Mn into the electrolytic solution, there remains a problem that the stability or cycle characteristics are not sufficient.
Also, LiFePO4 has been proposed but does not have sufficiently large capacity and has a problem that the discharge voltage is not high.
In this connection, it has been proposed to use LiMnPO4 based on Mn that is an element higher in oxidation-reduction potential than Fe, for the positive electrode of a lithium ion battery. However, a phosphoric acid compound having the basic composition of LiMnPO4 cannot easily realize redox generation of Mn. According to Journal of the Electrochemical Society, 144, 1188 (1997), among Mn-based phosphoric acid compounds, only LiMnxFe1-xPO4 having a structure formed by substituting Fe for a part of Mn is an example where redox generation of Mn is possible.
Furthermore, an electrode active material based on an oligophosphate represented by the following formula has been also proposed (Kohyo (Japanese Unexamined Patent Publication) No. 2006-523930):AaMbXcO(3c+1) (wherein (a) A is at least one alkali metal and 0<a≦6; (b) M is at least one redox active element and 1≦b≦4; (c) X is selected from the group consisting of P, As, Sb, Si, Ge, V, S and a combination thereof; (d) 2≦c≦5; and A, M, X, a, b and c are selected so as to keep electrical neutrality of the compound).
As specific examples, Li2FeP2O7, Na2CoP2O7, Na2NiP2O7, Na2MnP2O7, Li2Co0.5Ni0.5P2O7, Li2MnP2O7, Li2CoP2O7, Li2NiP2O7, Na2CuP2O7, Li2VP2O7, and Li0.5Na0.5FeP2O7 are disclosed.
According to the present inventors, when the amount of Fe is increased in Li2FeP2O7, a high battery capacity may be obtained, but it is difficult to synthesize the compound without impurities.